1. Field of the Invention
The present invention relates to a fracture splitting apparatus and a fracture splitting method for a fracture-split connecting rod, which are used when fabricating a part, particularly a connecting rod, used in an automobile engine or the like.
2. Description of the Related Art
A big end of a connecting rod used in an automobile engine or the like is split into a connecting rod body and a cap (a split half part on the opposite side of a small end) for assembly with a crank pin. As a method for splitting the big end, there has been proposed a manufacturing method of fracture splitting type for improving precision in positioning of the connecting rod body and the cap (Japanese Laid-open Patent Publication No. H03-014904).
In the conventional fracture splitting method, a pair of splitting jigs (hereinafter referred to as a mandrel) in a semi-cylindrical shape, which are movable in a direction of small end of the connecting rod and in a direction opposite thereto, is fitted into a big end of the connecting rod, and a wedge is driven between these jigs, to thereby split the connecting rod.
Here, a specific fracture splitting method which is performed conventionally will be described with reference to FIG. 8 to FIG. 10. In these views, first a connecting rod 100 has a big end 102 and a small end 103 on both ends of a connecting rod body 101. In the big end and the small end, there are formed a crank pin hole 104 and a piston pin hole 105, respectively. After being formed by forging, a cap 102A is integrated with the big end 102 as illustrated in FIG. 8, but the cap is separated from the connecting rod body 101 as illustrated in FIG. 10 by fracture splitting with the mandrel 200 of the fracture splitting apparatus. In addition, after being split, the cap 102A is fixed to the connecting rod body 101 by fastening with bolts 106 on both sides of the crank pin hole 104.
In the above-described case, the mandrel 200 is formed to be divided into a pair of semi-cylindrical bodies 200A, 200B, which oppose each other on their coupling faces 201, and is fitted entirely into the crank pin hole 104 of the connecting rod 100 in a cylindrical shape as the whole. A wedge hole 202 having a tapered face 202a for driving a wedge 1 of the fracture splitting apparatus is formed in an axial direction of the crank pin in a center portion of the mandrel 200. Here, the inclination angle of the tapered face 202a is denoted by θ1 (see FIG. 9B). On the other hand, in an inner peripheral face of the crank pin hole 104 of the connecting rod 100, for example a pair of V-shaped notch grooves 107 is formed in advance in the axial direction of the crank pin at opposite positions in a diametrical direction thereof. By driving the wedge 1 with a high pressure into the wedge hole 202 of the mandrel 200 fitted into the crank pin hole 104 (arrow A in FIG. 8), the big end 102 is fractured along the notch grooves 107.
However, when the mandrel 200 with simple semi-cylindrical shapes as described above is used for fracture splitting, plastic deformation as shown by arrows B in FIG. 10 occurs when the connecting rod is fractured. This plastic deformation causes, referring to FIG. 6, the vicinities of fractured portions to be in the form entering the sides of the crank pin hole 104 as shown by dotted arrows and dotted lines in the true circle shape of the crank pin hole 104 after being split. As a result, circularity after being fractured may be deteriorated. Further, normally there is a correlation between the circularity after being fracture split and the circularity after being assembled with a crank pin, and thus also the circularity after being assembled with the crank pin may be deteriorated. When the circularity of the crank pin hole 104 is deteriorated in this manner, this deterioration may cause increase in friction or seizure of a connecting rod bearing, and may even cause worsening of noise while the engine is operating due to the increased friction.